Rabbit cytochrome P-450 isozymes 2 and 5 are present in lung and liver. In the liver, but not the lung, the concentrations of these isozymes are increased by treatment of rabbits with phenobarbital. Homologues of isozymes 2 and 5 have been detected in lungs of mice, rats, hamsters, guinea pigs and monkeys. Although homologues of isozyme 2 are also present in livers of these species, hepatic homologues of isozyme 5 are not detected in any species except the hamster. However, treatment of hamsters with phenobarbital does not increase the concentration of this isozyme in liver. Rabbit liver expresses three forms of isozyme 2, two of which are also present in rabbit lung. These forms have been defined on the basis of restriction mapping and sequencing of cDNAs derived from mRNA isolated from liver and lung. A second form present in both tissues differs by 6 out of 491 amino acid residues. A third varient, which is present only in liver, differs at 11 and 15 positions from the other two. The expression of the three forms of isozyme 2 in liver appears to be under independent control with respect to induction by phenobarbital. Results with oligo-specific probes indicate that one form is not induced, one is induced initially and is then repressed, and the third under-goes induction only following prolonged treatment. Although the total mRNA for isozyme 2 in lung is not affected by phenobarbital, it is not clear whether or not the relative proportions of the two forms present are altered. Partial sequence analysis of cDNAs for isozyme 5 indicate that the pulmonary and hepatic forms are the same. Contrary to reports in the literature, we find that pulmonary mRNAs for isozymes 2 and 5 are confined entirely to the fraction that binds to oligo-dT. The reason for this difference appears to reside in the methods used for RNA isolation. We have found that great care must be exercised in order to isolate intact, poly-A mRNA from lung. A reproducible method, which involves centrifugation in cesium chloride followed by precipitation, has been developed for the purification of pulmonary mRNA in high yield.